Sending a self-destructing note or email is a good way to to make it difficult for someone to forward your message, saving it to a hard drive or stop a third party email server from keeping the message archived for years. The only way around for someone to copy a self-destructing email would be taking a screenshot, the message would still have to be associated with the sender to compromise your privacy, some of the services below make it difficult to make a readable screen grab.
OneShar.es: Allows you to compose a text only message on their servers via SSL, you are then given a unique URL that can be copied into any email message, IM or chatroom, after someone views the URL to read the message it will automatically self-destruct. i.e. erase itself from the server
PrivNote: Web service using SSL to send secure self-destructing notes without any registration needed. The text message will be made unavailable through the link after someone reads it once, there are no configuration options other than leaving your email address to be notified when someone reads the note.
QuickForget: Designed to compose an online note through a SSL connection from your browser to their severs and easily set it up to expire after a specific number of views or length of time after which your note will be purged from the database for ever.
OneTimeSecret: After creating a self-destructing note you will be given two links, one that will display the message once and another link for you that will inform you if the message has been read when you visit it. Optionally you can set up a password to protect the message.
BurnNote: Mobile phone app only for Android and iPhone, Burn Note displays a count down when the recipient opens a message and automatically destroys when it reaches zero, this guarantees that if someone only one person is able to read the data. You can send messages to other Burn Note users, an email address or get a link to your message that you can post or send via Instant Messenger.
StealthNotes: Message can have a maximum of views before self-destructing or a date can be set up for the message to be erased. Messages can be composed using text or HTML code, there is no SSL.
Crypt-A-Byte: Online dropbox that allows you to send PGP encrypted messages or send a self-destructing message that is erased after the recipient reads it. The message is encrypted in the browser and the passphrase never stored in the server, it is impossible for Crypt-A-Byte to read or decrypt your notes.